Large power plants that serve to generate electricity normally encompass a synchronous generator that is directly connected to the electric grid. The generator is driven by a turbine, which can be a gas turbine, a steam turbine or a water turbine.
As a result of the fact that the generator is connected directly to the grid, the speed of the generator is given and constant. A problematic aspect of this fact is that the optimal speed of the turbine is normally not the same as that of the generator, so that consequently a mechanical gearbox is often arranged between the turbine and the generator. This is particularly true of gas turbines, whose optimal operating speed is considerably higher that the grid frequency. In the case of water turbines having a low speed, at least a partial adaptation can be achieved by using a generator having the appropriate number of poles.
Such a gearbox is sensitive and expensive to produce whenever large quantities of power have to be transformed, in addition to which it requires an extraordinary amount of maintenance. Moreover, gearboxes cannot be employed in the highest power ranges, as a result of which the configuration of the turbine and its mode of operation have to be selected outside of the optimum range. Furthermore, the optimal speed of the turbine depends on the load present and the optimal efficiency at different loads can only be ensured if the turbine can be operated at different speeds. Unfortunately, this is not possible in the case of a rigid arrangement.
In order to circumvent this problem, German application DE 103 30 473 A1, for instance, describes the possibility of using a flexible electronic solution to replace the rigid connection involving a gearbox. The turbine is coupled directly to the synchronous generator but the latter is connected to the grid via a frequency converter (see FIG. 1 in DE 103 30 473 A1). A completely regulated voltage intermediate circuit converter (voltage source converter) having a controlled rectifier (converter on the machine side), a capacitive intermediate circuit and a controlled inverter (converter on the grid side) is used in order to efficiently uncouple harmonics between the generator and the grid.
In order to avoid excessive switching losses, the converter is operated in the square-wave mode, in which the switching frequency corresponds to the fundamental frequency. In this case, only the frequency between the input voltage and the output voltage can be varied, but not the amplitude (hence the name frequency converter). The amplitude of the voltage can be adapted by means of the excitation of the generator, as is commonly done in the classic arrangement with a direct connection between the generator and the grid.
The generated active power and reactive power can be controlled by means of the excitation of the generator and so can the phase shift between the generator and the voltages of the rectifier as well as between the inverter and the grid voltages. Even though DE 103 30 473 A1 describes these fundamental principles of the mode of operation of such a converter, this document does not indicate any specific strategy or structure for controlling the converter since various solutions are possible.